Hashing
-------

Internally, when Varnish stores content in the cache it stores the object together with a hash
key to find the object again. In the default setup this key is
calculated based on the content of the *Host* header or the IP address
of the server and the URL.

Behold the `default vcl`::

 sub vcl_hash {
     hash_data(req.url);
     if (req.http.host) {
         hash_data(req.http.host);
     } else {
         hash_data(server.ip);
     }
     return (hash);
 }

As you can see it first checks in `req.url` then `req.http.host` if it
exists. It is worth pointing out that Varnish doesn't lowercase the
hostname or the URL before hashing it so in theory having "Varnish.org/"
and "varnish.org/" would result in different cache entries. Browsers
however, tend to lowercase hostnames.

You can change what goes into the hash. This way you can make Varnish
serve up different content to different clients based on arbitrary
criteria.

Let's say you want to serve pages in different languages to your users
based on where their IP address is located. You would need some Vmod
to get a country code and then put it into the hash. It might look
like this.

In `vcl_recv`::

  set req.http.X-Country-Code = geoip.lookup(client.ip);

And then add a `vcl_hash`::

 sub vcl_hash {
   hash_data(req.http.X-Country-Code);
 }

As the default VCL will take care of adding the host and URL to the
hash we don't have to do anything else. Be careful calling
``return(hash)`` as this will abort the execution of the default VCL and
Varnish can end up returning data based on
more or less random inputs.
